I'm going to go for sedna mobile defense missions too. Because not only do you get rubedo, you also get alloy plate (also an often scarce resource) and on top of that probably one of the best ways to farm EXP, not too mention the mods you get.

I've gotten 250+ on Oro before, but that was killing every enemy, breaking every container and exploring every inch of the map. I've also gotten 0 doing the same thing, haha. Average would be about 60-100, but it may have dropped a bit since the last patch... I haven't played Sedna, so I can't say how good the yield is there.